My husband is considering purchasing a black suit in lieu of a tuxedo. Is it appropriate to wear a tuxedo shirt and bowtie (vest/cummerbund) with this suit to make it look more like a tuxedo or should he stick with a regular shirt and tie?

Jackie,
I really think any suit and tie is fine for formal nights, even if its not a black suit. U'll see all variations of "formal" wear from tuxes to sports jackets and ties. So, a nice suit and tie should do the trick no matter what the color.
Regards,
Kuki
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A dark suit, black shoes, white shirt and nice tie will fit right in for any but the most formal of occaisions. Unless you are doing an Atlantic crossing on the QEII, don't worry about it. Tummy-bund and bow tie won't make a suit a tux, don't waste the money.